Method of forming container with high-crystallinity sidewall and low-crystallinity base

ABSTRACT

A method of making a polyester container having an enhanced level of crystallinity in the sidewall while maintaining a low level of crystallinity in a thickened base portion. The container is particularly useful as a refillable container which can withstand higher caustic wash temperatures and exhibits reduced flavor carryover, or as a hot-fill container. According to the method, a sidewall-forming section of a preform is initially expanded, heated to contract and crystallize the same, and than reexpanded; a base-forming portion of the preform is shielded from the heat treatment and is expanded either before or after the heat treatment step.

RELATED APPLICATIONS

This application is a continuation of Ser. No. 08/652,985, filed May 24,1996, entitled METHOD OF FORMING CONTAINER WITH HIGH-CRYSTALLINITYSIDEWALL AND LOW-CRYSTALLINITY BASE (now U.S. Pat. No. 5,829,614 datedNov. 3, 1998), which is a division of Ser. No. 08/082,029 filed Jun. 30,1993 (now U.S. Pat. No. 5,520,877 dated May 28, 1996), which is acontinuation-in-part of Ser. No. 07/909,988 filed Jul. 7, 1992 (now U.S.Pat. No. 5,281,387 dated Jan. 25, 1994).

FIELD OF THE INVENTION

This invention relates to new and useful improvements in containers, andmore particularly to a method of forming a container having enhancedsidewall crystallinity and low base crystallinity. The container isparticularly adapted for use as a refillable carbonated beveragecontainer able to withstand higher caustic wash temperatures and exhibitreduced product flavor carryover, or as a hot fill container.

BACKGROUND OF THE INVENTION

The market for PET refillable carbonated soft drink (CSD) bottles hasenjoyed significant growth worldwide since its introduction in 1987 byContinental PET Technologies. These bottles have been commercializedthroughout much of Europe, Central and South America, and are now movinginto the Far East market.

Refillable bottles reduce the existing landfill and recycle problemsassociated with disposable plastic beverage bottles. In addition, arefillable bottle provides a safer, lighter-weight plastic container inthose markets, currently dominated by glass, where legislation prohibitsuse of non-returnable packages. The goal is to produce a refillablebottle having the necessary physical characteristics to withstandnumerous refill cycles, and which is still economical to produce.

Generally, a refillable plastic bottle must maintain its functional andaesthetic features over a minimum of 10 and preferably over 20 cycles orloops to be considered economically feasible. A loop is comprised of (1)an empty hot caustic wash followed by (2) contaminant inspection andproduct filling/capping, (3) warehouse storage, (4) distribution towholesale and retail locations and (5) purchase, use and empty storageby the consumer followed by eventual return to the bottler. This cycleis illustrated in FIG. 1. In an alternative cycle, the contaminantinspection occurs prior to the caustic wash.

Refillable containers must meet several key performance criteria toachieve commercial viability, including:

1. high clarity (transparency) to permit on-line visual inspection;

2. dimensional stability over the life of the container; and

3. resistance to caustic wash induced stress cracking and leakage.

A commercially successful PET refillable CSD container is presentlybeing distributed by The Coca-Cola Company in Europe (hereinafter “theprior art container”). This container is formed of a single layer of apolyethylene terephthalate (PET) copolymer having 3-5% comonomer, suchas 1,4-cyclohexanedimethanol (CHDM) or isophthalic acid (IPA). Thepreform, from which this bottle is stretch blow molded, has a sidewallthickness on the order of 5-7 mm, or about 2-2.5 times that of a preformfor a disposable one-way bottle. This provides a greater average bottlesidewall thickness (i.e., 0.5-0.7 mm) required for abuse resistance anddimensional stability, based on a planar stretch ratio of about 10:1.The average crystallinity in the panel (cylindrical sidewall sectionbeneath the label) is about 15-20%. The high copolymer content preventsvisual crystallization, i.e., haze, from forming in the preform duringinjection molding. Preform haze is undesirable because it producesbottle haze which hinders the visual on-line inspection required ofcommercial refill containers. Various aspects of this prior artcontainer are described in Continental PET Technology's U.S. Pat. Nos.4,725,464, 4,755,404, 5,066,528 and 5,198,248.

The prior art container has a demonstrated field viability in excess of20 refill trips at caustic wash temperatures of up to 60° C. Althoughsuccessful, there exists a commercial need for an improved containerthat permits an increase in wash temperature of greater than 60° C.,along with a reduction in product flavor carryover. The latter occurswhen flavor ingredients from a first product (e.g., root beer) migrateinto the bottle sidewall and subsequently permeate into a second product(e.g., club soda) on a later fill cycle, thus influencing the taste ofthe second product. An increase in wash temperature may also bedesirable in order to increase the effectiveness and/or reduce the timeof the caustic wash, and may be required with certain food products suchas juice or milk.

Thus, it would be desirable to increase the caustic wash temperatureabove 60° C. for a returnable bottle having a lifetime of at least 10refill trips, and preferably 20 refill trips, and to reduce the productflavor carryover. These and other objects are achieved by the presentinvention as set forth below.

SUMMARY OF THE INVENTION

In accordance with this invention, a method of forming a container isprovided having an enhanced level of sidewall crystallinity and a lowlevel of base crystallinity. The container has improved resistance tocaustic stress cracking, while maintaining a high level of transparency(clarity) and dimensional stability, and thus is particularly suitablefor refillable beverage bottles. The container has a lifetime of atleast 10 refill cycles and more preferably at least 20 refill cycles, atcaustic washing temperatures of above 60° C. The container exhibits areduction in flavor carryover of at least 20% over the previouslydescribed refillable CSD prior art container.

The method of forming the container includes a first expansion step inwhich a substantially amorphous polyester preform is at least partiallyexpanded into an intermediate article, followed by a heat treating stepin which the intermediate article is at least partially heated tocontract and crystallize the same, and then a second expansion step inwhich the contracted intermediate article is reexpanded to form thefinal container.

In a first method embodiment of the invention, a base-forming section ofthe preform is not expanded during the first expansion step, is notheated and remains substantially unchanged in crystallinity during theheat treating step, and is expanded without significant crystallinitychange during the second expansion step. In contrast, a sidewall-formingsection of the preform is expanded during the first expansion step todimensions substantially equal to or greater than the dimensions of thefinal container sidewall, heated to crystallize and contract the samebelow the dimensions of the final container during the heat treatingstep, and reexpanded during the second expansion step to the finaldimensions of the container sidewall. The relatively thinner containersidewall thus achieves a substantially higher percent crystallinity thanthe thicker base, which provides enhanced resistance to caustic stresscracking in both the sidewall and base.

In a second method embodiment, the base-forming section of the preformis expanded during the first expansion step, but is not heated duringthe heat treating step so that it maintains a low level of crystallinitycompared to the container sidewall. Again, the sidewall-forming sectionof the preform is expanded during the first expansion step to form anintermediate expanded sidewall with dimensions substantially equal to orgreater than the dimensions of the final container sidewall, theexpanded intermediate sidewall is then heated to crystallize andcontract the same below the dimensions of the final container sidewall,and then the contracted intermediate sidewall is expanded during thesecond expansion step to the final dimensions of the container sidewall.The thinner container sidewall thus achieves a substantially higherpercent crystallinity than the thicker base, which provides enhancedresistance to caustic stress cracking in both the sidewall and base.

The base-forming section of the preform is generally substantiallythicker than the sidewall-forming section and thus more resistant toheating (and resultant crystallization) during the heat treating step.In addition, it is preferred to localize or confine the heat treatmentto the intermediate sidewall, while the base-forming section (or base)is shielded to prevent heating thereof. In one preferred heat treatingstep, the intermediate container is heated by passing through a row ofheating elements and shielding elements move (or increase in size) toprotect the base-forming section (or base) as it moves upwardly with thecontracting sidewall. In addition, a contracting centering rod ispositioned within the contracting intermediate article, and the internalpressure within the intermediate article is controlled, to promoteuniform and controlled contraction thereof. In another preferred heattreating step, a cooling mechanism such as a movable water-cooled basecup remains in contact with the base-forming section (or base) toprevent heating thereof. Alternatively, a cooling mechanism directs acooling fluid (such as cold air) against the base-forming section (orbase) of the contracting article to prevent heating of the base. Inaddition, the relatively thicker neck and shoulder sections may beshielded to prevent heating thereof.

The resulting container has a highly oriented, relatively thin andhighly crystalline sidewall panel portion having at least 25% averagecrystallinity, and more preferably about 30 to 35% averagecrystallinity. The container base includes a thickened base portion oflow orientation and crystallinity, i.e., no greater than about 10%average crystallinity. The wall thickness of the thickened base portionis generally at least 3×, and more typically about 3 to 4× that of thepanel. Higher crystallinity levels in the panel allow higher washtemperatures, e.g., 65° or 70° C., but require longer processing times(to heat and cool the sidewall). A very high crystallinity level of 50%has been achieved. By “average” crystallinity is meant an average takenover the entire area of the respective container part, i.e., panel orthickened base portion.

In one embodiment, the container is a one-piece refillable pressurizedbeverage container with a free-standing base. The sidewall (inparticular the panel) has a wall thickness of about 0.5 to about 0.8 mm,and during the first expanding step the sidewall-forming section of thepreform is stretched at a planar stretch ratio of about 10-16:1 (i.e.,the thickness reduction ratio of the expanded intermediate sidewall tothe preform sidewall), and during the second expansion step thecontracted intermediate sidewall is stretched at a planar stretch ratioof about 7-15:1, and more preferably 9-11:1 (i.e., the thicknessreduction ratio of the final sidewall to the preform sidewall). Thecontainer has a champagne base with an upwardly radially increasingarcuate outer base wall, a lowermost chime, and a recessed central dome,the chime preferably having an average percent crystallinity of nogreater than about 10%, and more preferably about 2-8%, and the centraldome preferably having an average crystallinity of no more than about8%, and more preferably no more than about 2%.

Alternatively, the container may have a substantially thinner “footed”base including a hemispherical bottom wall with downwardly extendinglegs which terminate in lowermost supporting feet. The hemisphericalbottom wall includes radial ribs between the legs. A relatively thinouter portion of the base (including the ribs, legs and feet) preferablyhas an average crystallinity of at least about 10%, and more preferablyabout 15-20%, and a substantially thicker central portion of the bottomwall (without legs) has an average crystallinity of no more than about8%, and preferably no more than about 2%.

In still another embodiment, the improved resistance to stress crackingand dimensional changes at elevated temperatures makes the container ofthis invention particularly suitable as a hot-fill container.

These and other features of the invention will be more particularlydescribed by the following detailed description and drawings of certainspecific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a typical cycle or loopthrough which a refillable container must pass;

FIG. 2 is a schematic elevational view of a PET refillable 1.5-litercarbonated beverage bottle of this invention with a champagne base,partially broken away, and showing the varying wall thickness andaverage percent crystallinity at various positions along the bottle;

FIG. 3 is a schematic elevational view of a PET refillable 1.5-litercarbonated beverage bottle of this invention with a footed base,partially broken away, and showing the varying wall thickness andpercent crystallinity at various positions along the bottle;

FIGS. 4-7 are schematic illustrations of a first method embodiment ofthe invention wherein the base-forming section of the preform is notexpanded during the first expansion step, with FIG. 4 showing thepreform positioned in a blow mold, FIG. 5 showing the first expansionstep, FIG. 6 showing the heat treatment by infrared (IR) heatingelements and shielding elements around the base-forming section, andFIG. 7 showing the second expansion step to from a final container witha champagne base;

FIGS. 8-11 are schematic illustrations of a second method embodiment ofthe invention wherein the base-forming section is expanded during thefirst expansion step, with FIG. 8 showing the preform positioned in ablow mold, FIG. 9 showing the first expansion step which includesexpansion of the base, FIG. 10 showing the heat treating step in whichthe base is shielded, and FIG. 11 showing the second expansion step toform a final container with a champagne base;

FIGS. 12-13 are schematic profiles of the containers of FIGS. 4-11during the sequential method steps, with FIG. 12 showing the sequentialprofiles for the first method embodiment of FIGS. 4-7 (base not blownduring first expansion step), and FIG. 13 showing the sequentialprofiles for the second method embodiment of FIGS. 8-11 (base blownduring first expansion step);

FIGS. 14-15 are schematic profiles of two alternative footed containers,made according to the two previously defined method embodiments, withFIG. 14 showing sequential profiles for the first method embodiment inwhich the base-forming section is not expanded during the firstexpansion step, and FIG. 15 showing the sequential profiles for thesecond method embodiment in which the outer base is partially expandedduring the first expansion step;

FIG. 16 is an enlarged schematic of an alternative apparatus for heattreating in which the intermediate article is exposed to hot air from apair of blowers; the container shown has a champagne base and is madeaccording to the first method embodiment of the invention;

FIG. 17 is an enlarged schematic showing an alternative apparatus forheat treating which includes, in addition to hot air blowers, awater-cooled base cup to prevent heating of the base; the containershown has a champagne base and is made according to the second methodembodiment of the invention;

FIG. 18 is an enlarged schematic showing an alternative apparatus forheat treating which includes, in addition to infrared heating elementsdirected at the sidewall and movable shields for the base, a tube fordirecting cold air at the base to prevent heating thereof; the containershown has a footed base and is made according to the second methodembodiment of the invention; and

FIG. 19 is an enlarged schematic showing an alternative apparatus forheat treating, wherein radio frequency (RF) electrodes of variablelength are provided to selectively heat the sidewall of a container witha champagne base made according to the first method embodiment of theinvention.

DETAILED DESCRIPTION

Referring now to the drawings, and in particular FIG. 1, a commercialrefillable container must withstand numerous refill cycles whilemaintaining its aesthetic and functional features. A test procedure forsimulating such a cycle would be as follows. As used in thespecification and claims, the ability to withstand a designated numberof refill cycles without crack failure and/or with a maximum volumechange is determined according to the following test procedure.

Each container is subjected to a typical commercial caustic washsolution prepared with 3.5% sodium hydroxide by weight and tap water.The wash solution is maintained at the designated wash temperature,i.e., 60° C. or more, in accordance with this invention. The bottles aresubmerged uncapped in the wash for 15 minutes to simulate thetime/temperature conditions of a commercial bottle wash system. Afterremoval from the wash solution, the bottles are rinsed in tap water andthen filled with a carbonated water solution at 4.0±0.2 atmospheres (tosimulate the pressure in a carbonated soft drink container), capped andplaced in a 38° C. convection oven at 50% relative humidity for 24hours. This elevated oven temperature is selected to simulate longercommercial storage periods at lower ambient temperatures. Upon removalfrom the oven; the containers are emptied and again subjected to thesame refill cycle, until failure.

A failure is-defined as any crack propagating through the bottle wallwhich results in leakage and pressure loss. Volume change is determinedby comparing the volume of liquid the container will hold at roomtemperature, both before and after each refill cycle.

The container of FIG. 2, described below, can withstand at least 20refill cycles at a wash temperature of greater than 60° C. withoutfailure, and with no more than about 1.5% volume change after 20 cycles.The container also exhibits at least a 20% reduction in product flavorcarryover (compared to the prior art CSD bottle) as determined by gaschromatography mass spectrometer measurements.

FIG. 2 shows a PET refillable 1.5 liter carbonated beverage bottlehaving a relatively thick champagne base, made in accordance with thisinvention. The bottle 10 is a unitary blow-molded, biaxially-orientedhollow body having an open upper end 12, with external screw threads onneck finish 14 for receiving a screw-on cap (not shown), and a lowerclosed base 16. Between the neck finish and base is a substantiallyvertically-disposed sidewall 18 including an upper tapered shoulderportion 20, and a substantially cylindrical panel portion 22 (defined byvertical axis or centerline CL of the bottle). The champagne base 16 hasa central outwardly-concave dome with a center gate portion 24, aninwardly concave chime area 28 including a standing ring on which thebottle rests, and a radially increasing and arcuate outer base portion30 for a smooth transition to the sidewall 18. The chime is asubstantially toroidal-shaped area around the standing ring which isthickened to resist stress cracking. The dome and chime form a thickenedbase portion, which is about 3-4× the thickness of the panel 22, andhaving an average crystallinity of no greater than about 10%.Preferably, the gate 24 has no more than about 2% average crystallinityand the chime no more than 8% average crystallinity. The thickened baseportion resists heating (and thus crystallinization) during the heattreating step, as compared to the thinner sidewall panel 22. Above thechime, there is a thinner outer base portion of about 50-70% of thethickness of the thickened base portion and increasing in crystallinityup to its junction with the sidewall. The thinner outer base wallprovides improved impact resistance.

The 1.5 liter container of FIG. 2 is about 13.2 inch (335 mm) in heightand about 3.6 inch (92 mm),in (widest) diameter. The varying wallthickness along the bottle from the neck finish to the base is listed(in mm) in FIG. 2, along with the corresponding average percentcrystallinity. The varying crystallinity levels correspond to thecombined extent to which the bottle wall portion is stretched(strain-induced crystallization) and heated (thermal-inducedcrystallization). To maintain transparency, any thermal-inducedcrystallinity should be from low-temperature induced heat setting, e.g.,in contract with a mold at mold temperatures of 110-140° C. for PET. Thepercent crystallinity is determined according to ASTM 1505 as follows:

% crystallinity=[(ds−da)/(dc−da)]×100

where ds=sample density in g/cm³, da=density of an amorphous film ofzero percent crystallinity (for PET 1.333 g/cm³), and dc=density of thecrystal calculated from unit cell parameters (for PET 1.455 g/cm³)

A preform for making the container of FIG. 2 has a sidewall thickness ofabout 0.24 in (6.1 mm) and the sidewall panel 22 is stretched at anaverage planar stretch ratio of about 10:1. The planar stretch ratio isthe ratio of the average thickness of the panel-forming portion of thepreform to the average thickness of the panel in the bottle. A preferredplanar stretch ratio for polyester refill beverage bottles of about 0.5to 2.0 liters/volume is about 7-14:1, and more preferably about 8-13:1.The hoop stretch is preferably 3-3.6:1 and the axial stretch 2-3:1. Thisproduces a container sidewall panel with the desired abuse resistance,and a preform sidewall with the desired visual transparency. Thesidewall thickness and stretch ratio selected depends on the dimensionsof the specific bottle, the internal pressure (e.g., 2 atm for beer, 4atm for soft drinks), and the processing characteristics of theparticular material (as determined for example, by the intrinsicviscosity).

As illustrated in FIG. 2, the panel portion 22 of the container which isblown to the greatest extent has the highest average percentcrystallinity of 25-35%. The tapered shoulder 20, which is also expandedsubstantially more than the base 16, has an average percentcrystallinity of 20-30%. In contrast, the substantially thicker andlesser blown base 16 has 0-2% crystallinity in the central gate 24, 2-8%in the chime 28, and ranges therebetween in the dome 26. The outer base30 crystallinity ranges from that in the chime 28 (2-8%) to about 20-30%where the outer base meets the cylindrical panel 22. The neck finish 14is not expanded and remains substantially amorphous at 0-2%crystallinity.

Varying levels of crystallinity can be achieved by a combination ofexpansion (strain-induced) and heat-setting (thermal-induced).Generally, strain-induced crystallinity tends to be substantiallyuniform across the thickness of the particular layer, whilethermal-induced crystallinity may exhibit a gradient across the wall. Inthis invention, a high level of crystallinity at the inner and outersurfaces of the sidewall alone is sufficient for improved stress crackresistance. However, typically a substantially constant average level ofcrystallinity is achieved across the sidewall.

The blown container should be substantially transparent based on thepercent crystallinity as previously defined. Another measure oftransparency is the percent haze for transmitted light through the wall(H_(T)) which is given by the following formula:

H_(T)=[Y_(d)÷(Y_(d)+Y_(s))]×100

where Y_(d) is the diffuse light transmitted by the specimen, and Y_(s)is the specular light transmitted by the specimen. The diffuse andspecular light transmission values are measured in accordance with ASTMMethod D 1003, using any standard color difference meter such as modelD25D3P manufactured by Hunterlab, Inc. The container of this inventionshould have a percent haze (through the wall) of less than about 15%,preferably less than about 10%, and more preferably less than about 5%.

The following test was conducted which showed a reduction in flavorcarry-over for a 1.5-liter container of FIG. 2 having an averagecrystallinity level in the panel of 30-35% (container I), and thepreviously described prior art bottle of the same-size and shape havingan average crystallinity level in the panel of 15-20% (container II).

A model beverage stimulant was prepared comprising the following fourmaterials (common to beverage products) mixed in deionized water atconcentrations normal to beverage products:

(a) material A is a cyclohexane;

(b) material B is an aldehyde;

(c) material C is an ethyl compound in the 195-205 molecular weightrange; and

(d) material D is a simple hydrocarbon chain in the 130-140 molecularweight range.

The model beverage stimulant was poured into the sample bottles and heldfor six weeks at 110° F.

The sample bottles were then emptied out and subjected to a simulatedcommercial wash at 60° C. and 15 minutes in a 2% sodium hydroxidesolution. The bottles were then filled with a weak acetic acid solutionand held at 110° F. for another six weeks. Note that this wash procedureis specific to this carryover test and not intended to modify thepreviously defined refill cycle simulated test procedure.

At the end of the second six-week holding period, the solution wasdecanted into well sealed glass bottles and refrigerated until tested.Testing was performed using a Hewlett-Packard gas chromatographer 5890A.The sample bottles contained the following average remanents ofmaterials A-D as shown below in micrograms per liter:

Container I Container II (FIG. 2) (prior art) Material A 92 155 MaterialB 560 962 Material C 0.13 0.25 Material D 0.57 1.2

The container of this invention (container I) generally showed abouthalf the flavor carry-over of the known commercial bottle. Containersmade according to this invention with even higher levels ofcrystallinity exhibited still larger reductions in flavor carry-over.

The following test was conducted and showed an improvement indimensional stability at elevated wash temperatures of theabove-described container of this invention (container I), as comparedto the previously described prior art container (container II). Again,this specific testis for illustrative purposes and not meant to modifythe previously defined refill cycle simulated test procedure.

Generally, a commercially viable refillable PET bottle should have avolume change of no more than 1.5% in 20 loops in up to five years. Theshrinkage potential of such a commercial five-year 20-loop cycle inmoderate climates was simulated by using a five-hour emersion in a 2%sodium hydroxide solution at the below designated wash temperatures. Ateach of the three wash temperatures, the container of this invention(container I with 30-35% average crystallizion in the panel) showedsignificantly less volume change compared to the prior art container(container II). An increase in shrinkage was shown with increasing washtemperature; to accommodate the same, a container with a highercrystallinity may be used, i.e., above 30-35%. Generally, a highercrystallinity level increases the processing cost, including the time ofheat treating, so that the bottle is more expensive to produce.

WASH CONTAINER I CONTAINER II TEMP (° C.) (FIG. 2) (Prior Art) 60.0 0.6%1.1% 62.5 0.9% 1.8% 65.0 1.7% 4.2%

An alternative PET refillable 1.5 liter carbonated beverage bottle madein accordance with this invention is shown in FIG. 3, but having asubstantially thinner footed base. The bottle 110 is a unitaryblow-molded, biaxially-oriented hollow body having an open upper end112, with external screw threads on neck finish 114 for receiving ascrew-on cap (not shown), and a closed lower base 116. Between the neckfinish and base is a substantially vertically-disposed sidewall 118including an upper tapered shoulder portion 120, and a substantiallycylindrical panel portion 122 (defined by vertical axis or center lineCL of the bottle). The integral base 116 is a substantiallyhemispherical bottom wall 129 with downwardly extending legs 125 eachhaving a lowermost supporting foot 128 on which the container rests.Radiating ribs 130 extend between the legs 125 and form part of thehemispherical bottom wall 129. A central dome portion 124 of thehemispherical bottom wall, which does not include any legs and isrelatively thick, forms a thickened central base portion. A thinnerouter base portion 131 includes the legs 125, feet 128 and ribs 130. Thelegs, which are blown further than the hemispherical, bottom wall andthus tend to be relatively thinner than the ribs, include an inner legportion 126 adjacent the dome and an outer leg portion 127 between thefoot and sidewall of the container.

As shown in FIG. 3, the average percent crystallinity in the containersidewall varies according to the amount the bottle portion is blown andheated. The panel portion 122 which is blown to the greatest extent, hasthe highest average crystallinity of 25-35%. The tapered shoulder 120has the next highest average crystallinity of 20-30%. The unexpandedneck finish 114 is substantially amorphous at 0-2% averagecrystallinity. The base 116, which is blown substantially less than thesidewall 118, has 0-2% average crystallinity in the central dome 124,15-18% average crystallinity in the foot 128, 10-15% averagecrystallinity in the ribs 130 (between the legs), and 20-30% averagecrystallinity adjacent the junction with the sidewall. The crystallinityof the inner leg portion 126 would vary between that of the dome 124(0-2%) and the foot 128 (15-18%). The crystallinity of the outer legportion 127 would likewise vary between that of the foot 128 (15-18%)and the upper base (20-30%).

The substantially higher sidewall panel crystallinity in the containersof FIGS. 2 and 3, along with the substantially lower base crystallinity,provides the enhanced level of resistance to caustic wash induced stresscracking in both the sidewall and base. In addition, it provides areduction in flavor carryover when the container is filled withdifferent beverages on subsequent refill cycles. The contrasting levelsof sidewall and base crystallinity can be achieved by the following twopreferred methods of making the container.

A first method embodiment of the invention is shown in FIGS. 4-7. Inthis first embodiment, the base-forming section of the preform is notexpanded during the first expansion step, and the base-forming sectionremains substantially unchanged in dimensions (and crystallinity) untilthe second expansion step. While the process is illustrated for making acontainer with a champagne base, it can similarly be used to make acontainer with a footed base (see FIG. 14).

As shown in FIG. 4, a preform 50 is suspended from a rotating collectassembly 200 and positioned in a first mold unit 214. The collectassembly includes a collect 202 which engages a neck finish 54 of thepreform and an internal bore 204 for supplying fluid to the interior ofthe preform. The collect assembly further includes a pressure reliefvalve 206 for controlling the fluid pressure within the preform duringthe various expansion and contraction steps, and a movable stretch rod208 which enhances uniform expansion and contraction of the preform. Themold unit 214 includes a neck plate 216 which engages a flange justbelow the neck finish 54 on the preform, an upper mold body 218 havingan inner surface 219 for forming the sidewall of the intermediatecontainer, and a lower mold body 220 having an inner surface 221 forengaging a base-forming section of the preform (which is not expandedduring the first expansion step). The mold portions 216, 218 and 220,arekept at various temperatures for reasons described below. The preform50, includes a sidewall-forming section 58 and a lower base-formingsection 56. The sidewall-forming section 58 includes an upper taperedshoulder-forming section 60 and cylindrical panel-forming section 62.The base-forming portion 56 may include a thickened upper portion 64 andthinner lower portion 66. A preferred preform for making a refillcontainer is described in U.S. Pat. No. 5,066,528 granted Nov. 19, 1991to Krishnakumar et al., which is hereby incorporated by reference in itsentirety.

As shown in FIG. 5, during the first expansion step the preform 50 isstretch blown (via rod 208 and a pressurized fluid) to form a firstintermediate article 70 having an expanded upper shoulder portion 72,expanded cylindrical panel portion 74, and unexpanded base-formingportion 76. Thus, the unexpanded base-forming portion 76 issubstantially identical in dimensions and crystallinity to the preformbase-forming section 56 (section 56 may be slightly smaller in diameterto facilitate insertion into the lower mold 220). The preform is hot,e.g., 200° F. (except for the neck finish) when it enters the mold. Thepreform is cooled as it expands in the mold and the mold sections 216,218 and 220 are kept at different temperatures to control thecrystallinity in different portions of the intermediate article. Theneck plate 216 (engaging the neck finish) is kept cold (e.g., 40-70°F.), the upper mold body 218 (forming the sidewall) is kept hot(180-210° F.), and the lower mold body (engaging the base-formingsection) is kept warm (e.g., 150-180° F.). Thus, the neck finish is keptamorphous, and the base is kept warm (for later expansion) and with verylow (if any) crystallinity.

As shown in FIG. 6, the first intermediate article 70 remains on therotating collect 202 for the heat treating step and the article 70 isinserted into a heat treating unit 228 which includes an outer enclosure230 with an upper heat shield 232 to protect the amorphous neck finish.The enclosure 230 is an elongated chamber through which the intermediatearticle 70 passes and the shoulder and panel portions 72, 74 are exposedto heat (arrows 235) from series of infrared (IR) heating elements 234which cause the sidewall to contract and crystallize as it moves throughthe chamber to form contracted shoulder portion 82 and contracted panelportion 84 of a second intermediate article 80. The heat treatingtemperature may be in the range of 400-500° F. The base portion 76 isshielded from heat 235 by shielding elements 236 which move upwardlywith the contracting article as it passes through the chamber. Again,the base-forming portion 86 of the second intermediate article remainssubstantially unchanged in dimensions and crystallinity from thebase-forming portion 76 of the first intermediate article. To facilitateuniform contraction of the first intermediate article 70, the centeringrod 208 shortens by means of internal spring 209 and the increase ininternal pressure within article 70 (due to contraction) is relieved bya pressure relief valve 206 so that the article 70 remains centered andcontracts in a controlled and uniform manner.

As shown in FIG. 7, during the second expansion step the contractedintermediate article 80 is stretch blown to form the final container 10(see FIG. 2). The article 80 remains on the rotating collect 202 and isinserted into a second mold unit 240 which includes a neck plate 242,upper mold body 244 and lower mold body 246. Pressurized air is injectedthrough the collect into the article 80 to expand the shoulder, paneland base portions 82, 84 and 86 and form the corresponding portions 20,22, 16 of the container 10. The intermediate article 80 is cooled as itexpands in the mold and the mold sections 242, 244 and 246 are kept atdifferent temperatures to control the crystallinity in differentportions of the final container. For example, the neck plate 242 is keptcold (e.g., 40-70° F.), the upper mold body 244 is kept warm to relieveresidual stresses in the sidewall (e.g., 120-150° F.) and the lower moldbody 246 is kept cold to keep the base crystallinity low (e.g., 40-70°F.). The expanded shoulder and panel sections 20 and 22 thus achieve asubstantially higher crystallinity level than the base 16 whichoptimizes the caustic wash induced stress crack resistance of thecontainer.

FIG. 12 shows a series of container profiles which correspond to thesteps shown in FIGS. 4-7. Profile 1 shows the preform 50 of FIG. 4 withthe base-forming section 56. Profile 2 shows the first intermediatearticle 70 after the first expansion step of FIG. 5, with thesubstantially unexpanded base-forming section 76. Profile 3 shows thesecond intermediate article 80 after the heat treating step of FIG. 6,with the substantially unchanged base-forming section 86. Profile 4shows the final container 10 after the second expansion step of FIG. 7,with the expanded but low crystallinity and relatively thick champagnebase 16.

The profiles in FIG. 14 correspond substantially to those in FIG. 12 butillustrate the formation of a container 110 having a footed base (seeFIG. 3). The reference numbers in FIG. 14 correspond to similar elementsin FIG. 12 with the addition of “100”. Thus, in FIG. 14, profile 1 showsa preform 150 for a footed container having a base-forming section 156.Profile 2 shows a first intermediate article 170 after the firstexpansion step with a substantially unchanged base-forming section 176.Profile 3 shows a second intermediate article 180 after the heattreating step again having a substantially unchanged base-formingsection 186. Profile 4 shows the final container 110 after the secondexpansion step having a footed base 116. The footed container 110 can bemade in an apparatus similar to that shown in FIGS. 4-7 withcorresponding adjustments for the formation of a footed base as opposedto a champagne base.

FIGS. 8-11 are similar to FIGS. 4-7 but illustrate a second methodembodiment of this invention wherein the base-forming section of thepreform is expanded during the first expansion step. FIGS. 8-11illustrate the formation of a container having a champagne base,although the process may also be used for the formation of a containerhaving a footed base (see FIG. 15). For similar elements, the referencenumbers in FIGS. 8-11 correspond to those in FIGS. 4-7 with the additionof a “prime” notation.

Thus, FIG. 8 shows a preform 50′ on a rotating collect assembly 200′ andpositioned within a first mold unit 214′. The elements substantiallycorrespond to those shown in FIG. 4 except for the lower part of themold unit 214′ wherein an expanded champagne base is to be formed by thelower mold unit 220′ during the first expansion step.

FIG. 9 illustrates the expansion of preform 50′ into first intermediatearticle 70′ during the first expansion step. Again, centering rod 208′axially draws the preform 50′ and fluid is injected into the center ofthe drawn preform to radially expand the same against the inner walls ofthe mold unit 214′. In this second embodiment, the sidewall sections 72′and 74′ are again expanded to dimensions equal to or greater than thedimensions of the corresponding final container sidewall sections (20and 22 in FIG. 2). In addition, the base-forming section 56′ is expandedto substantially the same dimensions as the desired dimensions of thefinal container base (16 in FIG. 2). Thus, following the first expansionstep the preform base-forming section 56′ has been expanded to form achampagne base 76′ with a central gate portion 75′, a concave recess77′, a chime 78′ and an outer base portion 79′. Similar to the firstembodiment, the neck plate 216′ is cold (e.g., 40-70° F.) and the uppermold body is hot (e.g., 180-210° F.). However, because the base 76′ hasnow been expanded during the first expansion step, the lower mold 220′is cold (e.g., 40-70° F.) to prevent crystallization of the expandedbase.

FIG. 10 illustrates the heat treating step in which the firstintermediate article 70′ is contracted to form the second intermediatearticle 80′. Again, article 70′ is disposed on rotating collect 202′ andinserted within a heat treating unit 228′ which includes an outerenclosure 230′, an upper heat shield 232′, and a series of infraredheating elements 234′ which apply heat 235′ (e.g., 400-500° F.) to thearticle 70′ as it moves along the elongated heat treating chamber.Again, movable shields 236′ protect the base of the article. Followingthe heat treating step, the shoulder and panel sections 72′, 74′ havebeen contracted to form shoulder and panel portions 82′, 84′ of secondintermediate article 80′, and the expanded base 76′ remainssubstantially unchanged in dimensions and crystallinity to become base86′.

FIG. 11 shows the second expansion step in which contracted intermediatearticle 80′ is expanded to form the final container 10′ (same ascontainer 10 in FIG. 2). Again, pressurized air is inserted via collet202′ to expand the contracted shoulder and panel sections 82′,84′ andform the corresponding shoulder and panel sections 20, 22 of the finalcontainer 10′. Again, neck plate 242′ is cold (e.g., 40-70° F.) so theneck finish remains substantially amorphous, upper mold body 244′ iswarm (e.g., 120-150° F.) to relieve residual stresses in the shoulderand panel sections of the container, and lower mold body 246′ is cold(e.g., 40-70° F.) so the base 16 remains substantially low incrystallinity. The container base remains substantially unchanged indimensions and crystallinity during the second expansion step.

FIG. 13 shows the container profiles corresponding to the second methodembodiment (FIGS. 8-11) for a container having a champagne base. Thus,profile 1 shows the preform 50′ having base-forming section 56′. Profile2 shows the first intermediate article 70′ after the first-expansionstep having an expanded base 76′ Profile 3 shows the second intermediatearticle 80′ after the heat treating step having a substantiallyunchanged base 86′. Profile 4 shows the final container 10′ having acontracted sidewall but a substantially unchanged base section 16′.

Similarly, a footed base can be formed according to the second methodembodiment of FIGS. 8-11, as shown by the container profiles of FIG. 15.However, in this case a central thickened portion 176′ of the baseremains unchanged during the first expansion step while an upper baseportion 177′ is expanded to form an upper hemispherical bottom wallduring the first expansion step. Profile 1 shows the preform 150′ havinga base-forming section 156′. Profile.2 shows the first intermediatearticle 170′ with base 173′ after the first expansion step, having anexpanded outer base portion 177′ but maintaining a substantially thickerunexpanded central base-forming section 176′. Profile 3 shows a secondintermediate article 180′ with base 183′ after the heat treating step,wherein the central thickened base-forming section 186′ is substantiallyunchanged (compared to section 176′), but the expanded sidewall andexpanded outer base portion 187′ have contracted. Profile 4 shows thefinal container 110′ with base 116′ having a thick central hemisphericalbottom wall portion 124′ (same as 124 in FIG. 3) of very lowcrystallinity (i.e., less than 2%), and a thinner expanded (althoughless than the sidewall) section 131′ (same as 131 in FIG. 3) includinglegs, feet and ribs having a relatively high crystallinity (i.e., 10 to20%), although lower than the sidewall panel (i.e., 25% and above).

FIGS. 16-19 show alternative heat treating apparatus. FIGS. 16 shows thesame rotating collet 202, centering rod 208 and second intermediatearticle 80 of FIG. 6, with an alternative heat treating unit 256including an outer enclosure 258 and blowers 260 which emit hot air 261for heating the first intermediate article 70 to form the secondintermediate article 80. The thickened base section 86 resiststhermal-induced crystallization, although shielding elements may also beprovided as shown in FIG. 6.

FIG. 17 illustrates the rotating collet 202′, centering rod 208′, andsecond intermediate article 80′ of FIG. 10. A heat treating unit 256 isprovided which includes hot air blowers 260 for heating the sidewall anda water-cooled base cup 272 for engaging the base section 76′ as itmoves upwardly with the contracting sidewall and becomes base section86′ (of substantially the same dimensions and crystallinity). Thewater-cooled base cup 272 is mounted on a movable piston 273 so that itremains in continuous contact with the base as the sidewall contractsand the base moves upwardly. The cup includes an upper surface 274 whichengages the thickened base portion, and further includes a channel 276for water to remove heat (arrow 277) from the base cup.

FIG. 18 shows rotating collet 202′, centering rod 208′ and a heattreating unit 228′ including an outer enclosure 230′, upper shield 232′and inductance heating rods 234′ which apply heat (arrow 235′) to thesidewall of intermediate article 170′, and moveable shields 236′ forprotecting the base 176′. The first intermediate article 170′ is adaptedto form a footed container and after the first expansion step thethickened central portion 176′ remains unchanged but the outer baseportion 177′ has been expanded. First article 170′ contracts to formsecond intermediate article 180′, but the central base portion 176′ (andadjacent portions of outer base 177′) are cooled by a stream of cool air(arrow 283) provided by pipe 282 to prevent crystallization-andcontraction of the central base. The resultant second article 180′ has abase 183′ including thickened central portion 186′ and thinner upperbase portion 187′.

FIG. 19 shows rotating collet 202, centering rod 208, and secondintermediate article 80 similar to FIG. 6, but with heat treating unit266 including an outer enclosure 267 and a series of radio frequency(RF) electrodes 268 which shorten in length, as shown by arrows andphantom lines 269, as the first intermediate article 70 moves along thechamber and contracts to form the second intermediate article 80. The RFelectrodes 268 are supplied by RP power input 271 and inductor 270. Bysupplying localized heating only to the sidewall as the articlecontracts and moves through the heat treating chamber, heating of thebase section 76 (and 86) is eliminated or substantially reduced.

The thermoplastic polyester materials used in this invention arepreferably those based on polyalkyvlne, and in particular, polyethyleneterephthalate (PET). PET polymers are prepared by polymerizingterephthalic acid or its ester-forming derivative with ethylene. Thepolymer comprises repbeating units of ethylene terephthalate of theformula:

The present invention contemplates the use of copolymer of polyethyleneterephthalate in which a minor proportion, for example, up to about 10%by weight, of the ethylene terephthalate units are replaced bycompatible monomer units. Thus, an used herein “PET” means PEThomopolymer and PET copolymer of the grades suitable for makingcontainers, which are well known in the art. The glycol moiety of themonomer may be replaced by aliphatic or alicyclic glycols such ascyclohexanedimethanol (CHDM), trimethylene glycol, polytetramethyleneglycol, hexamethylene glycol, dodecamethylene glycol, diethylene glycol,polyethylene glycol, polypropylene glycol, propane-1,3-diol,butane-1,4-diol, and neopentyl glycol, bisphenols, and other aromaticdiols such as hydroquinone and 2,2-bis(4′-B-hydrox-ethoxyphenyl)propane. Examples of dicarboxylic acid moieties which may be substitutedinto the monomer unit include aromatic dicarboxylic acids such asisophthalic acid (IPA), phthalic acid, naphthalene-dicarboxylic acid,diphenyldicarboxylic acid, diphenoxyethanedicarboxylic acids, bibenzoicacid, and aliphataic or alicyclic dicarboxylic acids such as adipicacid, sebacic acid, azelaic acid, decanedicarboxylic acid andcyclohexanedicarboxylic acid. In addition, various multifunctionalcompounds such as trimethylolpropane, pentaerythritol, trimellitic acidand trimesic acid can be copolymerized with the polyethyleneterephthalate polymer.

The polyethylene terephthalate polymers may contain other compatibleadditives and ingredients which do not adversely affect the performancecharacteristics of the container, such as adversely affecting the tasteor other properties of products packaged therein. Examples of suchingredients include thermal stabilizers, light stabilizers, dyes,pigments, plasticizers, fillers, antitoxidants, lubricants, extrusionaids, residual monomer scavengers and the like.

The intrinsic viscosity (I.V.) effects the processability of thepolyester resins. Polyethylene terephthalate having an intrinsicviscosity of about 0.8 is widely used in the CSD industry. Resins forvarious applications may range from about 0.55 to about 1.04, did moreparticularly from about 0.65 to 0.85. Intrinsic viscosity measurementsare made according to the procedure of ASTM D-2857, by employing0.0050±0.0002 g/ml of the polymer in a solvent comprising o-chlorophenol(melting point 0° C.), respectively, at 30° C. Intrinsic viscosity(I.V.) is given by the following formula:

I.V. (ln(V_(Soln.)/V_(Sol.)))/C

where:

V_(Soln.) is the viscosity of the solution in any units;

V_(Sol.) is the viscosity of the solvent in the same units; and

C is the concentration in grams of polymer per 100 mls of solution.

The preform for making the high-transparency refill bottle of thisinvention should be substantially amorphous, which for PET means up toabout 10% crystallinity, preferably no more than about 5% crystallinity,and more preferably no more than about 2% crystallinity. Thesubstantially amorphous or transparent nature of the preform mayalternatively be defined by a percent haze (H_(T)) of no move than about20%, preferably no more than about 10%, and more preferably no more thanabout 5%. The substantially amorphous preform may be a single layer ormulti-layer (e.g., with barrier layers for O₂ resistance and/or CO₂retention) preform made according to well-known injection processes,such as those described in U.S. Pat. No. 4,710,118 granted Dec. 1, 1987to Kirshnakumar et al., which is hereby incorporated by reference in itsentirety.

During injection molding of the preform, the hot injected preform may bequenched to room temperature and then reheated to within the orientationtemperature range before the distension step, i.e., reheat stretch blowprocess. Alternatively, the hot injection molded preform may bepartially quenched and allowed to equilibriate within the orientationtemperature range prior to distending, i.e., integrated process. Thesubstantially amorphous preform is then expanded which producesorientation and crystallization in the sidewall of the container. Theextent of stretching can be varied depending on the desired shape andwall thickness of the blown container and is controlled by affixing therelative dimensions of the initial preform and the finished container.The distension step should be carried out in the molecular orientationtemperature range for the polyester material being employed. Generallyspeaking, molecular orientation of an orientable thermoplastic materialoccurs over a temperature range varying from just above the glasstransition temperature (that temperature or narrow temperature rangebelow which the polymer is in a glassy state) up to just below the melttemperature of the polymer. As a practical-matter, the formation oforiented containers is carried out in a much narrower temperature range,known as the molecular orientation temperature range. The reason forthis is that when the temperature is too close to the glass transitiontemperature, the material is too stiff to stretch in conventionalprocessing equipment. When the temperature is increased theprocessibility improves greatly, but a practical upper limit is reachedat or near the temperature at which large aggregates of crystallitescalled spherulites begin to form, because the orientation process isadversely affected by spherulite growth. For substantially amorphouspolyester material, the molecular orientation range is typically fromabout 20 to 65° F. (11 to 36° C.), and more preferably from about 30 to40° F. (17 to 22° C.), above the glass transition temperature of thepolyester material. Typical amorphous PET polymer, which has a glasstransition temperature of about 168° F. (76° C.), generally has anorientation temperature range of about 195° F. (91° C.) to about 205° F.(96° C.).

Other factors important in the manufacture of refillable polyesterbeverage bottles are described in U.S. Pat. No. 4,334,627 toKirshnakumar et al. granted Jun. 15, 1982, U.S. Pat. No. 4,725,464 toCollette granted Feb. 16, 1988, and U.S. Pat. No. 5,066,528 toKrishnakumar et al. granted Nov. 19, 1991, which are hereby incorporatedby reference in their entirety.

As a further alternative, a multilayer preform described in a commonlyassigned and copending Ser. No. 07/909,961, entitled “Multi-LayerRefillable Container, Preform And Method Of Forming Same,” filed byinventors Collette et al, on Jul. 7, 1992, and a continuation-in-partapplication thereof filed on the same date as this application can beused in combination with the process of this invention. In addition touse of a high-copolymer (4-6%) core layer between low-copolymer (0-2%)inner and outer layers, other multilayer containers may include barrier,high thermal stability, recycle or post-consumer PET, or other layers.

As previously described, the plastic container of this invention ispreferably made of polyethelene terephthalate (PET). However, otherthermoplastic polyester resins may be used. The materials, wallthicknesses, preform and bottle contours, and processing techniques mayall be varied for a specific end product, while still incorporating thesubstance of this invention. The container may be for other pressurizedor unpressurized beverages (such as beer, juice or milk), or fornon-beverage products. The benefits of the invention, for example theimproved stress crack resistance at elevated temperatures, may beparticularly suitable for use as a hot-fill container, such as describedin U.S. Pat. No. 4,863,046 to Collette et al. granted Sep. 5, 1989,which is hereby incorporated by reference in its entirety. Hot-fillcontainers typically must withstand elevated temperatures on the orderof 180-185° F. (the product filling temperature) and positive internalpressures on the order of 2-5 psi (the filling line pressure) withoutsubstantial deformation (i.e., volume charge of no greater than about1%).

Thus, although several preferred embodiments of this invention have beenspecifically illustrated and described herein, it is to be understoodthat variations may be made in the preform construction, materials, thecontainer construction and methods of forming the container withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A container comprising a substantiallytransparent, biaxially-oriented, free-standing, blow-molded polyesterbody, the body having a sidewall, with an upper tapered shoulder and asubstantially cylindrical panel, and a base, the base being a champagnebase having a thickened base portion comprising a central dome and achime, the thickened base portion having with a wall thickness at leastabout 3× greater than a thickness of the panel, the panel having anaverage crystallinity of at least 35% and the thickened base portionhaving an average crystallinity of no greater than about 10%.
 2. Thecontainer of claim 1, wherein the container can withstand at least 10refill cycles in a caustic wash at a temperature of greater than 60° C.without crack failure.
 3. The container of claim 1, herein the containercan withstand at least 20 refill cycles in a caustic wash at atemperature of greater than 60° C. without crack failure.
 4. Thecontainer of claim 2 or claim 3, wherein the container can withstand thedesignated refill cycles with a maximum volume change of about ±1.5%. 5.The container of claim 1, wherein the polyester is a homopolymer orcopolymer of polyethylene terephthalate (PET).
 6. The container of claim1 or claim 5, wherein the panel has an average wall thickness of about0.50-0.80 mm, and the thickened base portion has an average wallthickness of about 2.0-4.0 mm and an average crystallinity of no greaterthan about 10%.
 7. The container of claim 1, wherein the container is afree-standing biaxially-oriented pressurized PET container.
 8. Thecontainer of claim 1, wherein the container is a hot-fill container. 9.The container of claim 1, wherein the polyester is a bottle grade PET.10. The container of claim 1, wherein the container has a multilayersidewall including at least one layer of a material selected from thegroup consisting of barrier, high thermal stability, and recycle PET.11. The container of claim 1, wherein the container has an unstretchedneck finish and the sidewall is biaxially oriented.
 12. The container ofclaim 11, wherein the shoulder has an average crystallinity of at least20%.
 13. The container of claim 12, wherein the chime has an averagecrystallinity in the range of 2-8%.
 14. The container of claim 13,wherein the dome has an average crystallinity in the range of 0 to 2%.15. The container of claim 14, wherein the panel has a planar stretchratio of 7-14:1.
 16. The container of any one of claims 2-3, wherein thewash temperature is 65° C.
 17. The container of any one of claims 2-3,wherein the wash temperature is 70° C.
 18. The container of claim 1,wherein the average crystallinity in the panel is strain-induced. 19.The container of claim 1, wherein the average crystallinity in the panelis a combination of strain-induced and thermal-induced.
 20. Thecontainer of claim 1, wherein the panel has a percent haze of no greaterthan 10%.
 21. The container of claim 1, wherein the panel has a percenthaze of no greater than 5%.
 22. The container of claim 1, wherein theshoulder, panel and base have a percent haze of no greater than 10%. 23.The container of claim 1, wherein the shoulder, panel and base have apercent haze of no greater than 5%.
 24. The container of claim 16,wherein the container can withstand the designated refill cycles with amaximum volume change of about ±1.5%.
 25. The container of claim 17,wherein the container can withstand the designated refill cycles with amaximum volume change of about ±1.5%.